1. Field of the Invention
The present invention relates to a magnetic disc drive device, and more particularly to a labyrinth structure formed in a passage connecting the inside of a ball bearing as a bearing to the outside thereof.
2. Related art
In a magnetic disc drive device, a ball bearing is used for rotatably supporting the hub which holds a magnetic disc.
In some types of the magnetic disc drive devices, a labyrinth structure is formed in a passage connecting the inside of a ball bearing as a bearing to the outside thereof. The labyrinth structure blocks the flow of dust particles, such as oil mist, generated in the ball bearing, toward a magnetic disc held by the hub.
FIGS. 8 and 9 show the structures of conventional magnetic disc drive devices.
In FIG. 8, a frame 71 has a cylindrical part 71a at the central part. A shaft 72 is forcibly applied and fixed to the cylindrical part 71a of the frame. A stator core 73 is fastened to the outer side of the cylindrical part 71a.
The stator core 73 is provided with a plural number of protruded poles. A drive coil 74 is wound, by a preset number of turns, on the protruded poles of the stator core.
The inner rings of the ball bearings 75 and 76 are fastened to the shaft 72 extended from and above the frame 71. A hub 77 is forcibly put into and fixed to the ball bearings 75 and 76. The hub 77 is rotatably supported.
The hub 77 includes a cylindrical outer surface 82 and a flange portion 81 continuous to the outer surface 82. A magnetic disc is put on the flange portion 81, through the outer surface 82, and clamped thereon by a clamper fastened to the hub 77. In this state, the magnetic disc is rotatable together with the hub 77.
The underside of the flange portion 81 includes a side wall 81a. A ring-like drive magnet 79 is fastened to the inner side of the side wall 81a.
The inner surface of the drive magnet 79 faces the tops of the protruded poles of the stator core 73 in a state that a proper gap is present therebetween.
With this structure, when current fed to the drive coil 74 is switched depending on an angular position of the drive magnet 79, the hub 77 and the magnetic disc held by the hub 77 are turned at a predetermined speed.
Another small side wall 86, which is integral with the hub 77, is disposed around the side wall 81a of the hub 77, thereby forming a groove 83 shaped arcuate about the axial line of the shaft 72.
A flange 88 is formed on the outer surface of the frame 71. A raised part 84, which is arcuate about the axial line of the shaft 72, is formed inside the flange 88. The raised part 84 is inserted into the groove 83 with a gap therebetween, thereby forming a labyrinth structure.
The outer surface of the side wall 86 is disposed facing the inner side 85 of the cylindrical side wall of the frame 71, while being properly spaced from each other.
Reference numeral 94 designates a flexible board electrically connected to an external circuit.
In the conventional magnetic disc drive device of FIG. 8, as described above, the raised part 84 of the frame 71 is fit into the groove 83 of the hub 77, thereby forming a labyrinth structure.
The outer side of the small side wall 86 of the hub 77 is disposed facing the inner side 85 of the cylindrical side wall of the frame 71 while being properly spaced from each other, thereby forming a labyrinth structure.
The dust particles, such as oil mist, generated in the ball bearings 75 and 76 are shut off by the labyrinth structure, and do not reach the magnetic disc.
The opening of the upper end of the hub 77 is covered with a cap 80. By the cap, dust particles, which are generated by the ball bearings 5 and 6, are prevented from going outside through the top end opening of the hub 77.
The conventional magnetic disc drive device shown in FIG. 9 will be described.
The differences of the FIG. 9 device from the FIG. 8 device are:
1) The fixed shaft is integral with the frame 71. PA1 2) The ring-like drive magnet 79 is fastened to the inner side of the side wall 90 of the hub 77, with the yoke 78 inserted therebetween. PA1 3) The outer side of the side wall 90 of the hub 77 is disposed facing the inner side of the side wall 91 of the frame 71, while being spaced a gap G1 from each other. A stepped part 93, which is formed in the inner side of the side wall 91 of the frame 71, faces the lower end face of the side wall 90 of the hub 77, thereby forming a labyrinth structure.
The remaining structure of the FIG. 9 device is substantially the same as that of the FIG. 8 device. Hence, the same or equivalent portions are designated by the same reference numerals as FIG. 8.
Also in the device of FIG. 9, the outer side of the side wall 90 of the hub 77 is disposed facing the inner side of the side wall 91 of the frame 71 while being spaced the gap G1, thereby forming the labyrinth structure. The dust particles, such as oil mist, generated in the ball bearings 75 and 76 are shut off by the labyrinth structure, and do not reach the hard disc.
Another labyrinth structure is disclosed in Published Unexamined Japanese Patent Application No. Hei. 4-22297. In the publication, a seal member is attached to the end of the ball bearing as viewed in the axial direction. A contact member is disposed covering a part of the seal member. A gap formed between the seal member and the contact member, thus arranged, forms a labyrinth structure.
In each of the conventional magnetic disc drive devices of FIGS. 8 and 9, the labyrinth structure is formed in the outer portion of the hub. The devices have the following problems.
In the magnetic disc drive device of FIG. 8, the groove and the raised part must be formed in the outer portion of the hub. This makes the shape thereof complicated and hence increases the machining cost.
In the magnetic disc drive device of FIG. 9, the gap G1 between the hub and the frame is set to be as small as possible (preferably 0.1 mm or shorter).
To realize such a gap, the hub and the frame must be precisely machined. An error caused when the ball bearing and the hub are mounted makes the hub unsteady. Accordingly, the gap between the hub and the frame must be determined allowing for the unsteadiness of the hub. This necessitates a relatively large gap. The resultant labyrinth structure has an insufficient dust shut-off function.
The labyrinth structure disclosed in Published Unexamined Japanese Patent Application No. Hei. 4-22297, is constructed by using the seal member provided at the end of the bearing and the contact member disposed facing the seal member while covering a part of the latter. The dust shut-off effect by such a labyrinth structure is unsatisfactory.